This invention relates to zinc-air button cells, and is particularly concerned with the production of a substantially leak-proof zinc-air button cell embodying a novel sealing arrangement which substantially prevents or minimizes leakage of electrolyte, and which also results in minimal hydrogen evolution, such cells having extended storage life substantially without loss of efficiency, and exhibiting substantially no pressure buildup even when the cell is completely sealed. The invention is also concerned with a method of fabrication of such improved zinc-air button cells.
Zinc-air cells employed as power sources are known. Such cells are constructed of a zinc electrode and a catalyst or air electrode, with a separator positioned between the zinc electrode and the catalyst electrode, the cell containing electrolyte such as aqueous potassium hydroxide, which permits electrolyte ions to pass or be conducted through such separator but prevents electrode ions from migrating through such separator or membrane.
The sealing of zinc-air cells of the above general type is essentially a problem of providing an efficient seal which prevents any electrolyte creapage outwardly from the zinc anode material and the separator along the internal surfaces of the can or housing for the cell components. For this purpose such can surfaces must be rendered non-gassing when in contact with electrolyte and zinc, and any plastic seal employed must exert sufficiently large pressure to combat any electrolyte creapage by capillary action along the microscopic channels present on the metal can surfaces.
However, any pressure of a magnitude which will aid in sealing the anode side of the cell in most cases will create a leak path along the cathode seal. Polytetrafluoroethylene (PTFE) membranes which generally form the hydrophobic barrier usually employed in zinc-air systems, are dimensionally unstable under substantial pressure and such excessive pressures will destroy the permeability and render futile any attempt to seal the metal surfaces. On the other hand, any relaxation of such pressure creates capillary conditions permitting electrolyte to undesirably creap along the cathode can surface.
Creapage of electrolyte along surfaces with negative potential is characterized by hydrogen evolution on such surfaces. The gas bubbles tend to lift the seal and allow electrolyte to wet more of the surface on which in turn gas bubbles will form and allow electrolyte to penetrate even further.
In addition to the problem of providing a non-leaking zinc-air cell, there is also the problem of storage and active life during continuous or intermittent discharge. Prior art zinc-air cells exhibit a tendency to rapid deterioration when exposed to air. This deterioration often is due to the combined effect of rapid carbonation of the cathode, and premature oxidation of the zinc anode.
Accordingly, one object of the invention is to provide a substantially leak-proof metal-air cell, particularly a button cell designed for use in hearing aids. Another object is the provision of a substantially leak-proof zinc-air button cell for the above purpose. Another object is to provide a button cell as noted above, particularly a zinc-air cell, including a novel sealing arrangement substantially preventing leakage of electrolyte from the anode can of such bottom button Yet another object is to minimize gas, particularly hydrogen, evolution from the cell and to prevent pressure buildup when such cell is completely sealed. A further object is to provide a zinc-air cell having the above characteristics and which can be stored over relatively long periods of time with minimum loss of efficiency and capacity. A still further object is to provide relatively simple procedure for fabricating such improved, particularly zinc-air, button cells.